Fix datadir handling in bitcoind

This prevents premature tries to access or create the default datadir.
This is useful when the -datadir option is specified and the default
datadir is unreachable.

src/bitcoind.cpp

@@ -92,8 +92,7 @@ static bool AppInit(int argc, char* argv[])
 
     try
     {
-        if (!fs::is_directory(GetDataDir(false)))
-        {
+        if (!CheckDataDirOption()) {
             return InitError(strprintf("Specified data directory \"%s\" does not exist.\n", gArgs.GetArg("-datadir", "")));
         }
         if (!gArgs.ReadConfigFiles(error, true)) {